The invention involves a laboratory thermostat having a bath container for a liquid to be tempered as well as a refrigerating unit, wherein the refrigerating unit has a refrigeration cycle having a compressor, a condenser, an evaporator, as well as a mass flow regulator (expansion valve) controlled by a temperature sensor arranged at the outlet of the evaporator, and having a regulation device that is connected to a temperature value sensor for detecting the bath temperature, and, if necessary, having a heating unit for the bath liquid.
In laboratory thermostats of this type cost-effective membrane valves, which are also called thermostatic expansion valves, can be used as the expansion valve. A liquid thermometer, which is arranged at the outlet of the evaporator and measures the temperature in the refrigerant there, generally functions for controlling the membrane valve. Depending on the temperature difference between the intake and outlet of the evaporator, the mass flow is changed in such a way that the evaporator outputs its optimal refrigerating capacity. The temperature of the intake is indirectly measured by the pressure.
Using the thermostatic expansion valve, the refrigerating capacity is thus adapted to the actual requirements and limited accordingly. In order to achieve a thermostatic regulation of the liquids to be tempered in the bath container, i.e., keeping a constant liquid temperature previously set in the regulation device, it is known according to German published patent application DE 38 18 321 A1 to provide, in addition to the output limiter-expansion valve, a hot gas-bypass valve, which is in controlling connection with the regulation device. The bypass valve, which can be magnetically opened or closed, is inserted into a bypass line, which is connected on one end after the compressor and before the condenser and on the other end after the expansion valve and before the evaporator. Thus, the hot refrigerant gas flows through the evaporator when the hot gas-bypass valve is open. In this case, there will be no cooling. For regulation of the temperature of the bath liquid, the bypass valve is opened and closed at appropriate time intervals.
It is disadvantageous therein that the refrigeration capacity is not continuously available, but instead is switched between 0% and 100%. If a high regulation accuracy is necessary with this two point regulation, then to smooth out the temperature fluctuations, the bath volume must be large or the switching intervals must be done at shorter time intervals, which, however, greatly reduces the lifetime of the solenoid valve. Furthermore, during the switching operation large pressure jolts occur in the refrigeration cycle, which strain the components of the refrigeration cycle and can lead to a breakdown of these components, especially the expansion valve with a membrane.